It is well known in the art to improve the properties of pigments and fillers by applying organosilicon compounds on their surfaces. Such a coating process is intended to facilitate the incorporation and dispersion of pigments or fillers in lacquers, dispersion paints, plastics, toners, building materials, enamels, etc. In addition, the agglomeration of pigments and fillers during storage and processing is avoided by such a coating procedure with organosilicon compounds. It is furthermore desirable to reduce the accumulation of dust during the storage of pigments and fillers.
This state of the art is summarized in EP-A-0 373 426 (A2). EP-A-0 373 426 describes inorganic paint and magnetic pigments containing at least 0.1 percent by weight and at most 5 percent by weight of one or several polyorganosiloxanes, which have a viscosity of 100 to 100,000 mPa.times.sec and a relative molecular weight of 500 to 500,000, do not contain any reactive or cross-linking groups, and contain at least one Si-alkyl and/or Si-aryl group with 9 to 25 carbon atoms per molecule, these groups being present in the polyorganosiloxane in an amount up to 7 to 70 percent by weight and the remaining organic groups in the polyorganosiloxane having 1 to 8 carbon atoms.
Admittedly, the organosilicon compounds named in the aforementioned EP-A-0 373 426 are suitable in many cases for coating dye pigments. However, they frequently fail in the case of fillers, such as magnesium or aluminum oxides or their oxide hydrates. Frequently, a better dispersibility of the coated particles and a better adhesion of the coating to the surface of the particles are also desired.
The British Patent GB 2,244,489 claims a filler which is provided with a coating of cross-linked silicone elastomers linked chemically to the surface of the inorganic core. It is evident from the examples cited there that, to begin with, highly cross linked silicone elastomers with a plurality of reactive SiOH groups (also referred to as "rubbery reactive silicones") are synthesized first. These are then applied on the inorganic filler and must be cured on this filler for 4 hours at 250.degree. C. The silicone elastomer is the product of a cross-linking reaction between a "multifunctionally terminated" polysiloxane and a silane cross-linking agent. These coating materials are cross linked elastomers intended to improve mechanical properties, such as the extensibility and tenacity, of filled polymers.
The present invention is concerned with the technical problem of finding organosilicon compounds, which are suitable as universally as possible for coating finely divided particles of various types, particularly also of fillers. Moreover, the coated particles should have good application properties, such as an improved dispersibility and improved compatibility in organic media, such as lacquers, plastics, mineral oils, synthetic oils, vegetable oils or plasticizers. The coating shall adhere well to the surfaces of the particles so that permanence of the properties is ensured.
EP-A-0 373 426 teaches that the organopolysiloxane should be free of reactive groups. Surprisingly, it was found that the organopolysiloxanes show improved properties as coating materials on pigments and fillers especially when, in departure from the teachings of EP-A-0 373 426, groups which have a certain, graded reactivity, are linked in the siloxane molecule to silicon atoms.